Multimodal polyethylene material

ABSTRACT

The present invention relates to a polyethylene resin having a multimodal molecular weight distribution, said resin being further characterized in that it has a density in the range of from about 0.925 g/ccm to about 0.950 g/ccm, a melt index (I 2 ) In the range of from about 0.05 g/10 min to about 5 g/10 min, and in that it comprises at least one high molecular weight (HMW) ethylene interpolymer and at least a low molecular weight (LMW) ethylene polymer, and a composition comprising such resin. Also provided is a shaped article comprising said resin or composition, in particular a pipe.

The present application is a continuation application of U.S.Application No. 10/484,906, filed Aug. 10, 2004, now U.S. Pat. No.7,250,473, which is a 35 U.S.C. §371 National Stage of InternationalApplication No. PCT/US02/27503, filed on Aug. 28, 2002, which claims thebenefit of U.S. Provisional Application No. 60/316,401, filed Aug. 31,2001; each application is incorporated herein, in its entirety, byreference.

The present invention relates to a multimodal polyethylene resin, acomposition comprising such resin and to applications of such resin orcomposition, for example to make a shaped article. The resin andcomposition of the invention are particularly suitable for use in pipes.

Polyethylene compositions with a multimodal molecular weightdistribution (MWD), for example a bimodal MWD, can offer distinctadvantages compared with unimodal polyethylenes or other polyolefins.For example, bimodal polyethylenes may combine the favorable mechanicalproperties afforded by a high molecular weight polyethylene with thegood processability of a low molecular weight polyethylene. The priorart reports that such materials can advantageously be employed invarious applications, including film or pipe applications. Prior artmultimodal polyethylenes suggested for use in pipes include thematerials disclosed in the PCT applications with the publication numbersWO 97/29152, WO 00/01765, WO 00/18814, WO 01/02480 and WO 01/25328.

In view of the potentially disastrous consequences of material failures,acceptance of any plastic pipe for water or gas distribution is subjectto product standards and performance requirements set forth in norms,for example, DIN (German Instrustrial Norm or “Deutsche Industrie Norm”)or norms defined by ISO (International Organization for Standardization,Geneva, Switzerland). For example, state of the art polyethylenematerials sold into pipe applications, such as pressure pipes orirrigation pipes, meet the so-called PE80 or PE100 ratings (PE standsfor polyethylene). Pipes manufactured from polyethylenes classifying asPE80-type or PE100-type resins must withstand a minimum circumferentialstress, or hoop stress, of 8 MPa (PE80) or 10 MPa (PE100) at 20° C. for50 years. PE100 resins are high density polyethylene (HDPE) gradestypically having a density of at least about 0.950 g/ccm³ or higher.

Their relatively poor Long Term Hydrostatic Strength (LTHS) at hightemperatures has been an acknowledged disadvantage of traditionalpolyethylenes which rendered these materials unsuitable for use inpiping with exposure to higher temperatures, such as domestic pipeapplications. Domestic pipe systems typically operate at pressuresbetween about 2 and about 10 bar and temperatures of up to about 70° C.with malfunction temperatures of about 95-100° C. Domestic pipes includepipes for hot and/or cold water in pressurized heating and drinkingwater networks within buildings as well as pipes for snow melt or heatrecovery systems. The performance requirements for the various classesof hot water pipes, including underfloor heating, radiator connectorsand sanitary pipes are specified, for example, in International StandardISO 10508 (first edition Oct. 15, 1995, “Thermoplastic pipes andfittings for hot and cold water systems”).

Materials which are typically used for piping with exposure to highertemperatures include polybutylene, random copolymer polypropylene andcross-linked polyethylene (PEX). The crosslinking of the polyethylene isneeded to obtain the desired LTHS at high temperatures. The crosslinkingcan be performed during extrusion, resulting in lower output, or in apost extrusion process. In both cases crosslinking generatessignificantly higher costs than thermoplastic pipe extrusion.

Polyethylenes of Raised Temperature Resistance (PE-RT), as defined inISO-1043-1, are a class of polyethylene materials for high temperatureapplications which has recently been introduced to the pipe market.Present PE-RT resins compare unfavorably with PEX materials in somerespects, for example, in that the walls of PE-RT based pipes need to bethicker than those of PEX-based pipes due to lower stress ratings.

There still is the need for new polyethylene materials which offer anadvantageously balanced combination of thermal, mechanical andprocessing properties. In particular, there still is the need for newpolyethylene materials, which afford superior high temperatureresistance (e.g., in the range of operating temperatures from about 40°C. to about 80° C. and test temperatures of up to about 110° C.), highstress resistance, good tensile and impact performance and excellentprocessability without having to be crosslinked. It is an object of thepresent invention to meet these and other needs.

The present invention provides a polyethylene resin with a multimodalmolecular weight distribution. Said multimodal polyethylene resin ischaracterized in that it has a density in the range of from about 0.925g/ccm to about 0.950 g/ccm and a melt index in the range of from about0.05 g/10 min to about 5 g/10 min.

The present invention also provides a composition comprising suchmultimodal polyethylene resin and at least one other component.

Other aspects of the invention relate to applications of such multimodalpolyethylene resin and composition and to shaped articles made from suchpolyethylene resin or composition. One particular embodiment of thepresent invention relates to durable applications, such as pipes.

The term “comprising” as used herein means “including”.

The term “interpolymer” is used herein to indicate polymers prepared bythe polymerization of at least two monomers. The generic terminterpolymer thus embraces the terms copolymer, usually employed torefer to polymers prepared from two different monomers, and polymersprepared from more than two different monomers, such as terpolymers.

Unless indicated to the contrary, all parts, percentages and ratios areby weight. The expression “up to” when used to specify a numerical rangeincludes any value less than or equal to the numerical value whichfollows this expression. The expression “from to” when used to specify anumerical range includes any value equal to or higher than the numericalvalue which follows this expression. In these contexts, the word “about”is used to indicate that the specified numerical limit represents anapproximate value which may vary by 1%, 2%, 5% or sometimes 10%.

“HMW” stands for high molecular weight, “LMW” stands for low molecularweight.

The abbreviation “ccm” stands for cubic centimeters.

Unless expressly specified otherwise, the term “melt index” means the I₂melt index, as determined in accordance with ASTM D1238 under a load of2.16 kg and at a temperature of 190° C.

Unless specified otherwise, the term “alpha-olefin” (α-olefin) refers toan aliphatic or cyclo-aliphatic alpha-olefin having at least 4,preferably from 4 to 20 carbon atoms.

The present invention provides a polyethylene resin having a multimodalmolecular weight distribution, said resin being further characterized inthat it has

(a) a density in the range of from about 0.925 g/ccm, preferably of fromabout 0.935 g/ccm, to about 0.950 g/ccm, preferably to about 0.945g/ccm, and

(b) a melt index (I₂) in the range of from about 0.05 g/10 min,preferably of from about 0.1 g/10 min, to about 5 g/10 min, preferablyto about 1 g/10 min.

Said multimodal polyethylene resin comprises at least one high molecularweight (HMW) ethylene interpolymer and at least a low molecular weight(LMW) ethylene polymer. The HMW interpolymer has a significantly higherweight average molecular weight than the LMW polymer. Said difference inmolecular weight is reflected in distinct melt indices. Preferred is amultimodal polyethylene resin which has a trimodal or, most preferably,a bimodal molecular weight distribution. A bimodal polyethylene resinaccording to the present invention consists of one unimodal HMW ethyleneinterpolymer and one unimodal LMW ethylene polymer.

The HMW component characterizing the multimodal polyethylene resin ofthe invention comprises at least one or more, preferably one HMWethylene interpolymer. Such ethylene interpolymer is characterized by adensity in the range of from about 0.910 g/ccm, preferably of from about0.915 g/ccm, to about 0.935 g/ccm, preferably to about 0.925 g/ccm, anda melt index of about 1.0 g/10 min or lower, preferably of about 0.05g/10 min or lower. Advantageously, the HMW ethylene interpolymer has amelt index of about 0.02 g/10 min or higher. The HMW ethyleneinterpolymer contains ethylene interpolymerized with at least onealpha-olefin, preferably an aliphatic C₄-C₂₀ alpha-olefin, and/or anon-conjugated C₆-C₁₈ diolefin, such as 1,4-hexadiene or 1,7-octadiene.Although the HMW interpolymer can be a terpolymer, the preferredinterpolymer is a copolymer of ethylene and an aliphatic alpha-olefin,more preferably such an alpha-olefin which has from four to ten carbonatoms. Particularly preferred aliphatic alpha-olefins are selected fromthe group consisting of butene, pentene, hexene, heptene and octene.Advantageously, the HMW component is present in an amount of from about30 weight percent, preferably of from about 40 percent, to about 60weight percent, preferably to about 50 percent (based on the totalamount of polymer in the multimodal polyethylene resin). Morepreferably, the HMW component is present in an amount of from about 40to about 55 percent. The molecular weight distribution as reflected bythe M_(w)/M_(n) ratio of the HMW component is relatively narrow,preferably less than about 3.5, more preferably less than about 2.4.

The LMW component characterizing the multimodal polyethylene resin ofthe invention comprises at least one or more, preferably one LMWethylene polymer. The LMW ethylene polymer is characterized by a densityin the range of from about 0.945 g/ccm to about 0.965 g/ccm and a meltindex of at least about 2.0 g/10 min or higher, preferably of at leastabout 5 g/10 min, more preferably of at least about 15 g/10 min orhigher. Advantageously, the LMW component has a melt index of less than2000 g/10 min, preferably of less than 200 g/10 min. A preferred LMWethylene polymer is an ethylene interpolymer having a density in therange of from about 0.950 g/ccm to about 0.960 g/ccm and a melt index ofat least about 2 g/10 min, preferably in the range of from about 10 g/10min to about 150 g/10 min. Preferred LMW ethylene interpolymers areethylene/alpha-olefin copolymers, particularly such copolymers whereinthe aliphatic alpha-olefin comonomer has from four to ten carbon atoms.The most preferred aliphatic alpha-olefin comonomers are selected fromthe group consisting of butene, pentene, hexene, heptene and octene.Advantageously, the LMW component is present in an amount of from about40 weight percent, preferably of from about 50 percent, to about 70weight percent, preferably to about 60 percent (based on the totalamount of polymers comprised in the multimodal polyethylene resin of theinvention). More preferably, the LMW component is present in an amountof from about 45 to about 60 percent.

While the alpha-olefins incorporated into a HMW component and into a LMWcomponent comprised in a multimodal polyethylene resin of the inventionmay be different, preferred are such multimodal polyethylene resins,wherein the HMW and the LMW interpolymers incorporate the same type ofalpha-olefin, preferably 1-butene, 1-pentene, 1-hexene, 1-heptene or1-octene. Typically, the comonomer incorporation in the HMW ethyleneinterpolymer is higher than in the LMW polymer.

The multimodality of a polyethylene resin according to the presentinvention can be determined according to known methods. A multimodalmolecular weight distribution (MWD) is reflected in a gel permeationchromatography (GPC) curve exhibiting two or more component polymerswherein the number of component polymers corresponds to the number ofdiscernible peaks, or one component polymer may exist as a hump,shoulder or tail relative to the MWD of the other component polymer.

For example, a bimodal MWD can be deconvoluted into two components: theHMW component and the LMW component. After deconvolution, the peak widthat half maxima (WAHM) and the weight average molecular weight (M_(w)) ofeach component can be obtained. Then the degree of separation (“DOS”)between the two components can be calculated by the following equation:

${DOS} = \frac{M_{w}^{H} - M_{w}^{L}}{{WAHM}^{H} + {WAHM}^{L}}$

wherein M_(w) ^(H) and M_(w) ^(L) are the respective weight averagemolecular weight of the HMW component and the LMW component; andWAHM^(H) and WAHM^(L) are the respective peak width at the half maximaof the deconvoluted molecular weight distribution curve for the HMWcomponent and the LMW component. The DOS for the bimodal resinsaccording to the invention is at least 0.01 or higher, preferably higherthan about 0.05, 0.1, 0.5, or 0.8.

WO 99/14271 also describes a suitable deconvolution technique formulticomponent polymer blend compositions.

Preferably, the HMW component and the LMW component are each unimodal.The MWD in the GPC curves of the individual components, e.g. the HMWcomponent and the LMW component, respectively, does not substantiallyexhibit multiple component polymers (i.e. no humps, shoulders or tailsexist or are substantially discernible in the GPC curve). Each molecularweight distribution is sufficiently narrow and their average molecularweights are different. The ethylene interpolymers suitable for use asHMW and/or LMW component include both homogeneously branched(homogeneous) interpolymers and heterogeneously branched (heterogeneous)interpolymers.

Homogeneous ethylene interpolymers for use in the present inventionencompass ethylene-based interpolymers in which any comonomer israndomly distributed within a given interpolymer molecule and whereinall of the interpolymer molecules have substantially the sameethylene/comonomer ratio. Homogeneous ethylene interpolymers aregenerally characterized as having an essentially single melting (point)peak between −30° C. and 150° C., as determined by differential scanningcalorimetry (DSC). Typically, homogeneous ethylene interpolymers alsohave a relatively narrow molecular weight distribution (MWD) as comparedto corresponding heterogeneous ethylene interpolymers. Preferably, themolecular weight distribution defined as the ratio of weight averagemolecular weight to number average molecular weight (M_(w)/M_(n)), isless than about 3.5. Furthermore, the homogeneity of the ethyleneinterpolymers is reflected in a narrow composition distribution, whichcan be measured and expressed using known methods and parameters, suchas SCBDI (Short Chain Branch Distribution Index) or CDBI (CompositionDistribution Breadth Index). The SCBDI of a polymer is readilycalculated from data obtained from techniques known in the art, such as,for example, temperature rising elution fractionation (typicallyabbreviated as “TREF”) as described, for example, in Wild et al, Journalof Polymer Science, Poly. Phys. Ed., Vol. 20, p. 441 (1982), in U.S.Pat. No. 4,798,081 (Hazlitt et al.), or in U.S. Pat. No. 5,089,321 (Chumet al.), the disclosures of all of which are incorporated herein byreference. CDBI is defined as the weight percent of the polymermolecules having a comonomer content within 50 percent of the mediantotal molar comonomer content. The SCBDI or CDBI for the homogeneousethylene/alpha-olefin interpolymers used in the present invention istypically higher than about 50 percent.

The homogeneous ethylene interpolymers which can be used in the presentinvention fall into two categories, the linear homogeneous ethyleneinterpolymers and the substantially linear homogeneous ethyleneinterpolymers. Both are known in the art and commercially available.

Homogeneous linear ethylene interpolymers are interpolymers which have ahomogeneous short chain branching distribution and lack measurable ordetectable long chain branching. Such homogeneous linear ethyleneinterpolymers can be made using polymerization processes which provide auniform branching distribution, e.g., the process described by Elston inU.S. Pat. No. 3,645,992, who uses soluble vanadium catalyst systems.Other single-site catalyst systems including metallocene catalystsystems, e.g., of the type disclosed in U.S. Pat. No. 4,937,299 to Ewenet al., or U.S. Pat. No. 5,218,071 to Tsutsui et al., are also suitablefor the preparation of homogeneous linear ethylene interpolymers.

The substantially linear ethylene interpolymers (SLEPs) are homogeneousinterpolymers having long chain branching, meaning that the bulkethylene interpolymer is substituted, on average, with about 0.01 longchain branches/1000 total carbons to about 3 long chain branches/1000total carbons (wherein “total carbons” includes both backbone and branchcarbon atoms). Preferred polymers are substituted with about 0.01 longchain branches/1000 total carbons to about 1 long chain branches/1000total carbons, more preferably from about 0.05 long chain branches/1000total carbons to about 1 long chain branched/1000 total carbons, andespecially from about 0.3 long chain branches/1000 total carbons toabout 1 long chain branches/1000 total carbons. The presence of longchain branches in such ethylene interpolymers can be determinedaccording to methods known in the art, such as gel permeationchromatography coupled with a low angle laser light scattering detector(GPC-LALLS) and gel permeation chromatography coupled with adifferential viscometer detector (GPC-DV).

For substantially linear ethylene polymers, the presence of long chainbranching is manifest from enhanced rheological properties which can bequantified and expressed, for example, in terms of gas extrusionrheometry (GER) results and/or melt flow ratio (I₁₀/I₂) increases. Themelt flow ratio of the substantially linear ethylene/alpha-olefininterpolymers can be varied essentially independently of the molecularweight distribution (M_(w)/M_(n) ratio).

The substantially linear ethylene polymers are a unique class ofcompounds which has been described in numerous publications, includinge.g., U.S. Pat. No. 5,272,236, U.S. Pat. No. 5,278,272, and U.S. Pat.No. 5,665,800, each of which is incorporated herein by reference. SuchSLEPs are available, for example, from The Dow Chemical Company aspolymers made by the INSITE™ Process and Catalyst Technology, such asAFFINITY™ polyolefin plastomers (POPs).

Preferably, SLEPs are prepared using a constrained geometry catalyst.Such catalyst may be further described as comprising a metalcoordination complex comprising a metal of groups 3-10 or the Lanthanideseries of the Periodic Table of the Elements and a delocalized pi(π)-bonded moiety substituted with a constrain-inducing moiety, saidcomplex having a constrained geometry about the metal atom such that theangle at the metal between the centroid of the delocalized, substitutedpi-bonded moiety and the center of at least one remaining substituent isless than such angle in a similar complex containing a similar pi-bondedmoiety lacking in such constrain-inducing substituent, and providedfurther that for such complexes comprising more than one delocalized,substituted pi-bonded moiety, only one thereof for each metal atom ofthe complex is a cyclic, delocalized, substituted pi-bonded moiety.Suitable constrained geometry catalysts for manufacturing substantiallylinear ethylene polymers include, for example, the catalysts asdisclosed in U.S. Pat. No. 5,055,438; U.S. Pat. No. 5,132,380; U.S. Pat.No. 5,064,802; U.S. Pat. No. 5,470,993; U.S. Pat. No. 5,453,410; U.S.Pat. No. 5,374,696; U.S. Pat. No. 5,532,394; U.S. Pat. No. 5,494,874;and U.S. Pat. No. 5,189,192, the teachings of all of which areincorporated herein by reference.

The catalyst system further comprises a suitable activating cocatalyst.

Suitable cocatalysts for use herein include polymeric or oligomericaluminoxanes, especially methyl aluminoxane, as well as inert,compatible, noncoordinating, ion forming compounds. So-called modifiedmethyl aluminoxane (MMAO) is also suitable for use as a cocatalyst.Aluminoxanes, including modified methyl alunminoxanes, when used in thepolymerization, are preferably used such that the catalyst residueremaining in the (finished) polymer is preferably in the range of fromabout 0 to about 20 ppm aluminum, especially from about 0 to about 10ppm aluminum, and more preferably from about 0 to about 5 ppm aluminum.In order to measure the bulk polymer properties, aqueous HCl is used toextract the aluminoxane from the polymer. Preferred cocatalysts,however, are inert, noncoordinating, boron compounds such as thosedescribed in EP-A-0520732, the disclosure of which is incorporatedherein by reference.

Substantially linear ethylene interpolymers are produced via acontinuous (as opposed to a batch) controlled polymerization processusing at least one reactor (e.g., as disclosed in WO 93/07187, WO93/07188, and WO 93/07189, the disclosure of each of which isincorporated herein by reference), but can also be produced usingmultiple reactors (e.g., using a multiple reactor configuration asdescribed in U.S. Pat. No. 3,914,342, the disclosure of which isincorporated herein by reference) at a polymerization temperature andpressure sufficient to produce the interpolymers having the desiredproperties. The multiple reactors can be operated in series or inparallel, with at least one constrained geometry catalyst employed in atleast one of the reactors.

Substantially linear ethylene polymers can be prepared via continuoussolution, slurry, or gas phase polymerization in the presence of aconstrained geometry catalyst, e.g. according to the method disclosed inEP-A-416,815, the disclosure of which is incorporated herein byreference. The polymerization can generally be performed in any reactorsystem known in the art including, but not limited to, a tankreactor(s), a sphere reactor(s), a recycling loop reactor(s) orcombinations thereof and the like, any reactor or all reactors operatedpartially or completely adiabatically, nonadiabatically or a combinationof both and the like. Preferably, a continuous loop-reactor solutionpolymerization process is used to manufacture the substantially linearethylene polymer used in the present invention.

In general, the continuous polymerization required to manufacturesubstantially linear ethylene polymers may be accomplished at conditionswell known in the art for Ziegler-Natta or Kaminsky-Sinn typepolymerization reactions, that is, temperatures from 0 to 250° C. andpressures from atmospheric to 1000 atmospheres (100 MPa). Suspension,solution, slurry, gas phase or other process conditions may be employedif desired.

A support may be employed in the polymerization, but preferably thecatalysts are used in a homogeneous (i.e., soluble) manner. It will, ofcourse, be appreciated that the active catalyst system forms in situ ifthe catalyst and the cocatalyst components thereof are added directly tothe polymerization process and a suitable solvent or diluent, includingcondensed monomer, is used in said polymerization process. It is,however, preferred to form the active catalyst in a separate step in asuitable solvent prior to adding the same to the polymerization mixture.

Heterogeneous ethylene-based polymers encompass ethylene/α-olefininterpolymers characterized as having a linear backbone and a DSCmelting curve having a distinct melting point peak greater than 115° C.attributable to a high density fraction. Such heterogeneousinterpolymers typically have a broader molecular weight distributionthan comparable homogeneous interpolymers. Typically, heterogeneousethylene interpolymers have a CDBI of about 50% or less, indicating thatsuch interpolymers are a mixture of molecules having differing comonomercontents and differing amounts of short chain branching. Theheterogeneous ethylene polymers that can be used in the practice of thisinvention include those prepared with a coordination catalyst at hightemperature and relatively low pressure. Ethylene polymers andcopolymers prepared by the use of a (multi-site) coordination catalyst,such as a Ziegler-Natta catalyst or a Phillips catalyst, are generallyknown as linear polymers because of the substantial absence of branchchains of polymerized monomer units pendant from the backbone.

The HMW ethylene interpolymer can be a heterogeneous interpolymer or ahomogeneous interpolymer, a homogeneous interpolymer being preferred.Particularly preferred HMW ethylene interpolymers are homogeneous,substantially linear HMW ethylene interpolymers. The LMW ethyleneinterpolymer can be a heterogeneous interpolymer or a homogeneousinterpolymer, a heterogeneous interpolymer being preferred.

The multimodal polyethylene resin of the invention may be prepared byany method suitable for homogeneously blending ethylene-based polymers.For example, the HMW and the LMW component can be blended by mechanicalmeans in the solid state, for example, in powder or granular form,followed by melting one or both, preferably both of the components,using devices and equipment known in the art. Preferably, the multimodalresin of the present invention is made by in-situ blending of the HMWcomponent with the LMW component, e.g. using two or more reactors,operated sequentially or in parallel. According to a preferredtechnique, the multimodal polyethylene resin of the invention is madevia the interpolymerization of ethylene and the desired comonomer orcomonomers, such as an aliphatic C₄-C₁₀ alpha-olefin using a single sitecatalyst, e.g. a constrained geometry catalyst in at least one reactorand a multi site catalyst in at least one other reactor. The reactorscan be operated in parallel or, preferably, sequentially. Preferably,the single site catalyst, e.g. the constrained geometry catalyst is inthe first reactor, and the multi site catalyst in the second reactor.

Most especially, a dual sequential polymerization system is used. In apreferred embodiment of the invention, the sequential polymerization isconducted such that fresh catalyst is separately injected in eachreactor. Preferably, where separate catalyst injection into each reactoris, no (or substantially no) live polymer or active catalyst is carriedover from the first reactor into the second reactor as thepolymerization in the second reactor is accomplished only from theinjection of a fresh catalyst and monomer (and comonomer) thereto.

In another preferred embodiment, the composition is manufactured using amultiple reactor system (preferably a two reactor system) in series withflesh catalyst feed injection of a soluble catalyst system into thefirst reactor only with process adjustments being made such that livepolymer and/or catalyst species is carried over from the first reactorto a subsequent reactor to effect polymerization with fresh monomer andoptionally comonomer.

Most preferably, whether separate injection into each reactor is used orinjection into the first reactor is used, the resulting resin ischaracterized as comprising component polymers having distinct, unimodalmolecular weight distributions.

Most preferred is a multimodal polyethylene resin comprising a HMWinterpolymer designated herein as preferred, more preferred orparticularly preferred and a LMW polymer designated herein as preferred,more preferred or particularly preferred, including the bimodalpolyethylene resin used to exemplify the present invention.

The present invention also provides compositions comprising themultimodal polyethylene resin of the invention and at least one otheradditional component. Preferably, such additional component is added tothe multimodal polyethylene resin of the invention. Suitable additionalcomponents include, for example, other polymers, fillers oradditives—with the proviso that these additional components do notadversely interfere with the desired advantageous properties of themultimodal polyethylene resin of the invention. Rather, the additionalcomponents are selected such as to support the advantageous propertiesof the multimodal ethylene resin of the invention and/or to support orenhance its particular suitability for a desired application. Otherpolymers comprised in the composition of the invention means polymerswhich do not qualify as a HMW interpolymer or a LMW polymer as definedherein. Advantageously, such polymers are compatible with the multimodalpolyethylene resin of the invention. Preferred additional components arenon-polymeric. Additives include processing aids, UV stabilizers,antioxidants, pigments or colorants. Most preferred are compositionscomprising a preferred, more preferred or most preferred multimodalpolyethylene resin of the invention.

The resins or compositions of the present invention can be used tomanufacture a shaped article. Such article may be a single-layer or amulti-layer article, which is obtainable by suitable known conversiontechniques applying heat, pressure or a combination thereof to obtainthe shaped article. Suitable conversion techniques include, for example,blow-molding, co-extrusion blow-molding, injection molding, injectionstretch blow molding, compression molding, extrusion, pultrusion,calendering and thermoforing. Shaped articles provided by the inventioninclude, for example, films, sheets, fibers, profiles, moldings andpipes. Most preferred is a shaped article comprising or made from apreferred, more preferred or particularly preferred resin or compositionof the present invention.

The multimodal polyethylene resins and compositions according to thepresent invention are particularly suitable for durable application,especially pipes—without the need for cross-linking. Pipes comprising atleast one multimodal polyethylene resin as provided herein are anotheraspect of the present invention and include monolayer pipes as well asmultilayer pipes, including multilayer composite pipes. Typically, thepipes of the invention comprise the multimodal polyethylene resin inform of a composition (formulation) which also contains a suitablecombination of additives, e.g. an additive package designed for pipeapplications, and/or one or more fillers. Such additives and additivepackages are known in the art.

Monolayer pipes according to the present invention consist of one layermade from a composition according to the present invention comprising amultimodal polyethylene resin as provided herein and suitable additivestypically used or suitable for pipe applications. Such additives includecolorants and materials suitable to protect the bulk polymer fromspecific adverse environmental effects, e.g. oxidation during extrusionor degradation under service conditions, such as, for example, processstabilizers, antioxidants, pigments, metal de-activators, additives toimprove chlorine resistance and UV protectors. Preferred multilayercomposite pipes include metal plastic composite pipes and are pipescomprising one or more, e.g., one or two, layers comprising acomposition according to the present invention and a barrier layer. Suchpipes include, for example, three-layer composite pipes with the generalstructure PE/Adhesive/Barrier or Barrier/Adhesive/PE, or five-layerpipes with the general structure PE/Adhesive/Barrier/Adhesive/PE orPolyolefin/Adhesive/Barrier/Adhesive/PE. In these structures PE standsfor polyethylene layers which can be made from the same or differentpolyethylene compositions, preferably a PE-RT comprising composition,including at least one multimodal polyethylene composition according tothe present invention. Suitable polyolefins include, for example, highdensity polyethylene, polypropylene and polybutylene, homopolymers andinterpolymers. Preferred is a multilayer composite pipe wherein at leastthe inner layer comprises a multimodal polyethylene resin according tothe present invention in a non-crosslinked form. More preferred is amultilayer composite pipe, wherein both PE layers comprise a multimodalpolyethylene resin according to the present invention. In multilayerpipes, e.g. in the three-layer and five-layer structures exemplifiedabove, the barrier layer may be an organic polymer capable of providingthe desired barrier properties, such as an ethylene-vinyl alcoholcopolymer (EVOH), or a metal, for example, aluminum or stainless steel.

The resins and compositions provided by the present invention areparticularly suitable for use in domestic and technical pipeapplications required to be operable at higher temperatures, e.g. above40° C., in particular in the range of from above 40° C. to about 80° C.Such pipe applications include, for example, hot water pipes, e.g. fordrinking and/or sanitary purposes and underfloor heating pipes. Suchpipes may be monolayer or multilayer pipes. Preferred pipes according tothe invention meet the performance requirements as defined in the normsfor hot water pipes, e.g. in ISO 10508. The multimodal polyethyleneresin according to the present invention enables pipes combining anexcellent high temperature performance, as reflected e.g. in anexcellent Long Term Hydrostatic Strength at higher temperatures (wellabove 20° C.) with good flexibility. Good flexibility facilitates e.g.pipe installation. The pipes can be produced without cross-linking,which allows improved processing economics and subsequent welding.

For plastic pipe applications, circumferential (hoop) stress performanceas set forth in ISO 9080 and ISO 1167 is an important requirement. Thelong term behaviour or lifetime of plastic pipes can be predicted basedon creep rupture data and curves which establish the allowable hoopstress (circumferential stress) which a pipe can withstand withoutfailure. Typically, for long term predictive performance testing,candidate pipe materials are subjected to various pressures (stresses)and the lifetime at a given temperature is determined. Forextrapolations to a lifetime of 50 years, e.g. at 20° C. to 70° C.,testing is also performed at higher temperatures. The measured lifetimecurves at each temperature typically comprise a high stress, lowerlifetime ductile failure mode and a lower stress, longer lifetimebrittle failure mode. A schematic representation of typical lifetimecurves is found at page 412, FIG. 5, of the publication by J. Scheirs etal., TRIP 4 (12), 1996, pages 408-415. The curves can be divided intothree stages, stage I representing the ductile failure stage, stage II(knee) representing a gradual change in failure mode from ductile tobrittle, and stage III representing the brittle failure stage. Ofparticular interest are stages II and III, because these stages controlthe lifetime of a pipe in practice. The pipes of the present inventionshow an excellent hoop stress performance particularly at highertemperatures.

The invention is further illustrated by the following Examples, which,however, shall not be construed as a limitation of the invention.

EXAMPLES

Melt indices are expressed as I₂ (determined according to ASTM D-1238,condition E, 190° C./2.16 kg). The ratio of I₁₀ (measured according toASTM D-1238, Condition N, 190° C./10 kg) to I₂ is the melt flow ratioand designated as I₁₀/I₂.

Tensile properties, such as yield stress, yield strain, maximum tensilestress and maximum elongation, stress at break and strain at break aredetermined in accordance with ISO 527 with test specimen 5A at a testspeed of 50 mm/min.

Izod impact properties are measured according to ASTM D-256.

Flexural modulus is measured according to ASTM D-790 and averagehardness D is determined according to ASTM D-2240.

The multimodal polyethylene resin used in the experiments is a bimodalethylene interpolymer having an I₂ of 0.85 g/10 min, a density of 0.940g/ccm and an I₁₀/I₂ of 9.8. The resin is made by in-situ blending using(continuous) solution process technology and two sequentially operatedreactors. The HMW ethylene interpolymer is a homogeneous, substantiallylinear ethylene/octene copolymer which is made in the primary reactorusing a constrained geometry catalyst. Said HMW interpolymer has an I₂of 0.034 g/10 min and a density of 0.921 g/ccm. The weight averagemolecular weight is 228,000 and the Mw/Mn ratio is 2.1. The LMW ethylenepolymer is a heterogeneous, linear ethylene/octene copolymer having anI₂ melt index of 20 g/10 min and a density of 0.953 g/ccm. The weightaverage molecular weight of the LMW polymer is 52,100 and the Mw/Mnratio is 3. The LMW ethylene polymer is made in the secondary reactorusing a multi-site Ziegler-Natta (coordination) catalyst. The ratio ofHMW copolymer to LMW copolymer in the bimodal polyethylene resin is 40to 60.

The resin has the following tensile, impact and other properties (eachgiven value represents the average of five measurements):

Yield stress [MPa]: 21 Yield strain [%]: 13 Maximum tensile stress [MPa]36 Maximum elongation [%] 760 Stress at break [MPa] 36 Strain at break[%] 760 Flexural modulus [MPa] 955 Hardness D 61 Izod at 20° C. [J/m]238 Izod at −40° C. [J/m] 8

Monolithic pipes made from the above resin are subjected to hydrostaticpressure testing using the test method described in ISO 1167 (1996) andwater as the internal and external test medium. The pipes have nominaldimensions of 16 mm×2 mm.

The hoop stress results are given in Table 1.

Temperature Hoop Stress Failure time [° C.] [MPa] [h]* Failure Mode* 2010.57 >3096 20 10.54 >10344 20 10.44 >10344 20 10.40 >4056 2010.32 >4056 80 5.65 656 ductile 80 5.59 1245 ductile 80 5.52 >5952 805.49 >3600 80 5.45 >3600 80 5.42 >5952 80 5.35 >4056 80 5.34 >3600 805.30 >5952 80 5.25 >3600 110 2.91 >3912 110 2.89 >3912 110 2.84 >2616110 2.79 >3912 110 2.47 >11976 110 2.11 >11976 *“>” indicates that thespecimen is still under test without failure. In such cases, no failuremode can be indicated.

The pipes made from the bimodal polyethylene resin show an excellenthoop stress performance, especially at high(er) temperatures.Surprisingly, no knee (stage II) reflecting a change in failure modefrom ductile to brittle is manifest, yet. Test results already go beyondthe control points for PE-RT according to DIN 16883 (1.9 MPa/8760 h at110° C.) and PEX according to ISO 10146 (2.5 MPa/8760 h at 110° C.).

1. A polyethylene resin having a multimodal molecular weightdistribution, said resin being further characterized in that it: (a) hasa density in the range from 0.935 g/ccm to 0.945 g/ccm, and (b) has amelt index (I₂) in the range from 0.1 g/10 min to 1.0 g/10 min, and (c)comprises 30 to 50 weight percent of a high molecular weight (HMW)component, and 70 to 50 weight percent of a low molecular weight (LMW)component; said weight percents being based on the total amount ofpolymer in the multimodal polyethylene resin, and wherein the HMWcomponent comprises at least one high molecular weight ethyleneinterpolymer having a density in the range from 0.915 g/ccm to 0.935g/ccm, and a melt index of 1.0 g/10 min or lower, and wherein the LMWcomponent comprises at least one low molecular weight ethylene polymerhaving a density in the range from 0.945 g/ccm to 0.960 g/ccm, and amelt index in the range from 2.0 g/10 min to less than 200 g/10 min, andwherein the at least one high molecular weight interpolymer is ahomogeneous ethylene interpolymer that has a single melting pointbetween −30° C. and 150° C., and a CDBI greater than 50 percent; and theat least one low molecular weight ethylene polymer is a heterogeneousethylene interpolymer that has a linear backbone, a DSC melting pointgreater than 115° C. attributed to a high density fraction, and a CDBIof 50 percent or less.
 2. The polyethylene resin of claim 1, wherein thelow molecular weight ethylene polymer has a density in the range from0.950 g/ccm to 0.960 g/ccm, and a melt index in the range from 10 g/10min to about 150 g/10 min.
 3. The polyethylene resin of claim 1, whereinthe low molecular weight ethylene polymer has a melt index in the rangefrom 2 g/10 min to 20 g/10 min.
 4. The polyethylene resin of claim 1,wherein the high molecular weight component is present in an amount from40 to 50 weight percent, based on the total amount of polymer in themultimodal polyethylene resin, and the low molecular weight component ispresent in an amount from 60 to 50 weight percent, based on the totalamount of polymer in the multimodal polyethylene resin.
 5. A compositioncomprising a polyethylene resin and at least one other additionalcomponent, and wherein the polyethylene resin has a multimodal molecularweight distribution, and has the following properties: (a) a density inthe range from 0.935 g/ccm to 0.945 g/ccm, and (b) a melt index (I₂) inthe range from 0.1 g/10 min to 1 g/10 min, and wherein the polyethyleneresin comprises 30 to 50 weight percent of a high molecular weight (HMW)component, and 70 to 50 weight percent of a low molecular weight (LMW)component; said weight percents being based on the total amount ofpolymer in the multimodal polyethylene resin, and wherein the HMWcomponent comprises at least one high molecular weight ethyleneinterpolymer having a density in the range from 0.915 g/ccm to 0.935g/ccm, and a melt index of 1.0 g/10 mm or lower, and wherein the LMWcomponent comprises at least one low molecular weight ethylene polymerhaving a density in the range from 0.950 g/ccm to 0.960 g/ccm, and amelt index in the range from 2.0 g/10 min to less than 200 g/10 min, andwherein the at least one low molecular weight ethylene polymer is aheterogeneous ethylene interpolymer that has a CDBI of 50 percent orless.
 6. The composition according to claim 5, wherein the at least oneother additional component is selected from the group consisting offillers and additives.
 7. The composition of claim 5, wherein the atleast one high molecular weight interpolymer is a homogeneous ethyleneinterpolymer that has a CDBI greater than 50 percent.
 8. The compositionof claim 5, wherein the low molecular weight ethylene polymer has a meltindex in the range from 2 g/10 min to 20 g/10 min.
 9. The composition ofclaim 5, wherein the high molecular weight component is present in anamount from 40 to 50 weight percent, based on the total amount ofpolymer in the multimodal polyethylene resin, and the low molecularweight component is present in an amount from 60 to 50 weight percent,based on the total amount of polymer in the multimodal polyethyleneresin.
 10. The composition of claim 5, wherein the high molecular weightinterpolymer has a molecular weight distribution (Mw/Mn) less than about3.5.
 11. A shaped article comprising the composition of claim
 5. 12. Theshaped article according to claim 5, wherein the article is a pipe. 13.A shaped article comprising at least one component formed from thecomposition of claim
 5. 14. The shaped article of claim 13, wherein theshaped article is selected from the group consisting of pipes, films,sheets, fibers, profiles and moldings.
 15. A polyethylene resin having amultimodal molecular weight distribution, said resin being furthercharacterized in that it: (a) has a density in the range from 0.925g/ccm to 0.950 g/ccm, and (b) has a melt index (I₂) in the range from0.1 g/10 min to 5 g/10 min, and (c) comprises a high molecular weight(HMW) component and a low molecular weight (LMW) component, and whereinthe HMW component comprises at least one high molecular weight ethyleneinterpolymer having a density in the range from 0.910 g/ccm to 0.935g/ccm, and a melt index of 1.0 g/10 min or lower, and wherein the LMWcomponent comprises at least one low molecular weight ethylene polymerhaving a density in the range from 0.945 g/ccm to 0.965 g/ccm, and amelt index in the range from 2.0 g/10 min to less than 200 g/10 min, andwherein the at least one high molecular weight interpolymer is ahomogeneous ethylene interpolymer that has a single melting pointbetween −30° C. and 150° C., and a CDBI greater than 50 percent; andwherein the at least one low molecular weight ethylene polymer is aheterogeneous ethylene interpolymer that has a linear backbone, a DSCmelting point greater than 115° C., attributable to a high densityfraction, and a CDBI of 50 percent or less.
 16. A composition comprisingthe polyethylene resin of claim 15 and at least one other additionalcomponent.
 17. The composition of claim 16, wherein the low molecularweight ethylene polymer has a density in the range from 0.950 g/ccm to0.960 g/ccm, and a melt index in the range from 10 g/10 min to about 150g/10 min.
 18. The composition of claim 16, wherein the low molecularweight ethylene polymer has a melt index in the range from 2 g/10 min to20 g/10 min.
 19. The composition of claim 16, wherein the high molecularweight interpolymer has a molecular weight distribution (Mw/Mn) lessthan about 3.5.
 20. A shaped article comprising the composition of claim16.
 21. A shaped article comprising at least one component formed fromthe composition of claim
 16. 22. The shaped article of claim 21, whereinthe shaped article is selected from the group consisting of pipes,films, sheets, fibers, profiles and moldings.